This application claims the priority of German patent document 196 45 897.8, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a vehicle which has at least four wheels, which are arranged on two axles, can be cushioned relative to the vehicle body in the direction of a vertical axis of the body and are provided with inflatable tires, and which also has position generators which are assigned to the wheels and generate signals that provide an indication or determination of the respective vertical distances between the wheel centers and the body.
Free-moving motor vehicles having suspension systems of this generic type are known. The position generators may monitor, for example, the spring lifts of the wheels, and set shock absorbers assigned to the wheels to an increased damping effect when large spring lifts occur, as are typical of bad roads.
In addition, the position generators can be used in vehicles with a level control system or with a variable clearance from the ground for controlling support assemblies assigned to the wheels.
German patent document DE 39 20 165 A1 discloses an arrangement which examines whether a vehicle wheel clearly deviates in its height from a virtual plane which is given by the position of three other wheels on two axles (for example a plane which contains the wheel centers). In the event of an extreme deviation of a wheel from such a plane, there will be no position correction of the vehicle body under given driving conditions, because such deviations typically occur when one vehicle wheel is driven over a considerable elevation, for example, the edge of a curb, or over a pronounced depression. As a rule, it is not useful in such cases for an automatic level control system to intervene.
German patent document DE-OS 23 47 793, discloses an arrangement which takes into consideration, in the case of a spring arrangement with a level control system, extreme height deviations of a wheel from a level predetermined by the other wheels when controlling the carrying capacity distribution of the wheels.
These known systems take advantage of the fact that the position of a plane is determined by three points which are spaced away from one another, and which together form the corner points of a (true) triangle. Correspondingly, the position of three wheels on two axles defines a plane relative to the vehicle body, and it is therefore possible to determine whether the additional wheel or additional wheels take up a position which matches this plane.
It is an object of the invention to provide a new and improved arrangement for analyzing the signals of the position generators.
This object is achieved by the detection arrangement according to the invention, in which for each wheel, time-related mean values are determined for the distance between the wheel and the vehicle body and, from the mean values of three wheels on two axles, a desired center position of an additional wheel is then determined. A warning signal for an unequal tire pressure is generated if the actual center position of the additional wheel deviates excessively (that is, by more than a given tolerance) from the desired center position.
The invention is based on the general idea of utilizing the fact that, on a flat road, if the wheels and the tires are of the same type for each axle and have the same air pressure, the wheel centers of the wheels of two axles must be situated in a plane. The position of this plane relative to the body of the vehicle can be determined from the signals of the position generators.
Since the mentioned plane is already determined by the position of three wheels on two axles, a desired position for the fourth wheel can be determined from the positions of these three wheels, which desired position would have to be taken up by this wheel relative to the vehicle body if the ground is flat. Larger deviations of the actual position from the desired position indicate with an almost certain probability that the tire pressure of the wheels is unequal.
Although a completely even road can rarely be expected in practice, it may, as a rule, be assumed that uneven road conditions can be compensated by taking the time-related mean of the signals of the position generators; that is, the road can be "levelled" by the above-mentioned taking of the mean so that, as a result, an unequal tire pressure remains determinable at the tires of the wheels of one axle.
According to a preferred embodiment of the invention, the taking of the mean can be made more reliable by analyzing the signals of the position generators only above a predetermined limit speed which as a rule is reached or exceeded only in the case of comparatively good roads.
In addition to improving the measuring accuracy, this approach also takes into account the fact that unequal inflation of the tires of one axle has a negative influence on the driving safety only at higher driving speeds.
Because a warning signal can also be generated only above a predetermined threshold speed, the driver may drive to a gas station or repair shop at a moderate speed in order to correct the pressure of the tires, or carry out a tire change, without being irritated by a warning signal.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.